1. Field
Embodiments of the present invention generally relate to shallow trench isolation structures and methods for fabrication thereof.
2. Description of the Related Art
One challenge of fabricating, or etching, shallow trench isolation (STI) features in a substrate is microloading between regions of dense features and regions of isolated features. As illustratively shown in FIG. 4A, microloading manifests itself as differences in feature profile and etch depth between regions of high feature density 404 and regions of low feature density 406 on a substrate 402 in which the features are being etched. For example, regions of low feature density 406 may etched to a depth A that is different (greater, in the example of FIG. 4A) than an etch depth B corresponding to the regions of high feature density 404. Controlling microloading is important, as certain applications (such as NAND flash) require high microloading, while other applications (such as DRAM) require low to minimal microloading.
In addition, as line widths continue to shrink, the ion energies necessary to etch the dense features to a comparable etch depth and profile as compared to isolated features needs to be increased. However, an increase in bias power supplied to the substrate to increase the ion energy leads to other complications. For example, topography related asymmetric profile and floating gate polysilicon attack in the dense regions are some unintended consequences of using processes with high bias power. FIG. 4B depicts an illustrative example of an asymmetric feature profile where a sidewall 408 of a feature 410 being etched into the substrate 402 is not symmetrically disposed with respect to an opposing sidewall 412 of the feature 410. This problem may occur while providing a similar mask critical dimension (CD), as shown in FIG. 4B, or with a varying mask CD, as shown in FIG. 4C.
In another example, conventional STI etching chemistries, such as mixtures of hydrogen bromide, chlorine, and oxygen, typically require high ion energies (e.g., obtained from high bias power) to etch silicon. However, these high bias power processes often undesirably result in various defects, such as pinching off at high aspect ratios, poor microloading, and asymmetric feature profiles, particularly in regions of high feature density. FIG. 4D illustratively depicts an example of pinching off on a substrate 402 where a bottom 414 of the feature 410 being etched into the substrate 402 closes, or is pinched off, by inwardly sloping sidewalls 416 of the feature 410.
Moreover, other difficulties also exist. For example, achieving a good feature profile without any bowing or asymmetry in the STI structure or achieving very low etch depth range (good etch depth uniformity) is also difficult for certain conventional etch chemistries. Another problem encountered is the incompatibility of certain materials with conventional etch chemistries. For example, carbon-based masks used to define the STI are incompatible with the use of chlorine or oxygen as major gas species due to mask loss resulting from the attack of the mask material by these gases.
As such, conventional STI etch chemistries and processes suffer from deficiencies such as etch depth microloading, profile pinching off, asymmetric profiles (or profile distortion) related to pattern density and/or aspect ratio differences in the substrate being etched.
Thus, there is a need for fabricating STI structures defining high aspect ratio STI profiles that may provide at least one of reduced distortion, improved control over etch depth microloading, or mask integrity.